The present invention relates to a production process of intermediate starting materials used for perfumery, more specifically, 2-cyclohexenyl methyl ketones (1b) and 1-cyclohexenyl methyl ketones (1c), or mixtures thereof.
2,6,6-Trimethylcyclohexenyl methyl ketone is a useful compound as an intermediate for the synthesis of fruity floral fragrant materials such as xcex1- and xcex2-damascone. Damascones typified by xcex1-damascone which serves as a key note component of natural roses have been commercially produced and practically used as an important fruity floral fragrance. Damascones have three double-bond-depending isomers as described below. These Isomers have their own odor notes, respectively, while they have a fruity floral note basically. They are used differently according to their application purposes. 
A large number of production processes for these damascones {xcex4-damascone (2a), xcex1-damascone (2b), xcex2-damascone (2c)} have been reported, for example, in xe2x80x9cReviewxe2x80x9d (Shigeru Torii, et al., Koryo, No. 125, 47-60 (1979)). Among them, a production process of xcex1-Damascone (2b), xcex2-Damascone (2c), xcex4-Damascone (2a) represented by the following reaction 
wherein any one of dotted lines means a double bond, which was reported by K. Subrahmania {J. C. S. Perkin 1, 1, 727(1975)}, is known as a process focusing on to the Aldol reaction of 2,6,6-trimethylcyclohexenyl methyl ketone (1).
2,6,6-Trimethyl-3-cyclohexenyl methyl ketone (1a), which is one of the starting materials of the above-described reaction, is synthesized by the Diels-Alder reaction between 1,3-pentadiene (4) and mesityl oxide (5) as shown in the following reaction scheme: 
On the other hand, 2,6,6-trimethyl-2-cyclohexenyl methyl ketone (1b) is prepared by the following process, that is, by the Diels-Alder reaction of isobutene (6) and 4-methyl-3,5-hexadien-2-one (7). 
2,6,6-Trimethyl-1-cyclohexenyl methyl ketone (1c) is synthesized by the following reaction of ethylene (8) with 3-acetyl-2,4-dimethyl-1,3-pentadiene (9). 
However, a process for synthesizing methyl-substituted-cyclohexenyl methyl ketones represented by the formulas (1b) and (1c) by isomerization of the double bond of the compound represented by the formula (1a) is not known yet.
Since xcex1- and xcex2-damascones which are typical fruity-floral fragrant materials are expensive, there is a demand for the development of a low-cost production process therefore. The above-described production process of damascones by Subrahmania has a merit in its fewer production steps based on the Aldol reaction of 2,6,6-trimethylcyclohexenyl methyl ketones {(1b) and (1c)} with acetaldehyde. It is however difficult to industrially produce 4-methyl-3,5-hexadien-2-one (7) and 3-acetyl-2,4-dimethyl-1,3-pentadiene (9), the starting materials for the production of 2,6,6-trimethylcyclohexenyl methyl ketones {(1b) and (1c)}. Production of them also needs high costs. The above-described process is therefore not used industrially from the economical viewpoint. An object of the present invention is therefore to provide a more economical process for producing 2,6,6-trimethylcyclohexenyl methyl ketones {(1b) and (1c)}, that is, intermediates for the synthesis of xcex1- and xcex2-damascones.
Under the above-described conditions, the present inventors have carried out an extensive studies and, as a result, it has been found that methyl-substituted cyclohexenyl methyl ketones represented by (1b) and/or (1c), which are starting materials for the synthesis of xcex1- and/or xcex2-damascone, can be prepared economically by isomerization of methyl-substituted cyclohexenyl methyl ketone represented by the formula (1a).
In the present invention, there is thus provided a process for producing a 2-cyclohexenyl methyl ketone represented by the formula (1b), a 1-cyclohexenyl methyl ketone represented by the formula (1c), a trans-3-cyclohexenyl methyl ketone represented by the formula (1axe2x80x2), or a mixture thereof, which comprises isomerizing, in the presence of an isomerizing catalyst, a 3-cyclohexenyl methyl ketone represented by the formula (1a). 
wherein, R1, R2 and R3 each independently represents a hydrogen atom or a methyl group and at least two of R1, R2 and R3 represent a methyl group.